1. Field of the Invention
The present invention relates to a switched reluctance motor to be mounted to motor vehicles and applied as a driving motor driving a compressor built in refrigerators and air conditioning systems.
2. Description of the Related Art
Switched reluctance motors which operate without magnets are well known.
A description will be given of a conventional switched reluctance motor with reference to FIG. 26 to FIG. 28.
FIG. 26 is a vertical cross section of a conventional three phase switched reluctance motor in its axial direction. FIG. 27 shows a transverse cross section of the conventional three phase switched reluctance motor taken along an alternate long and short dash line O-O of FIG. 26. FIG. 28 is a schematic development elevation of stator magnetic poles of a stator, in a rotational direction of the rotor, facing magnetic poles of a rotor in the three phase switched reluctance motor shown in FIG. 26.
The stator 1 has a plurality of stator magnetic poles. Each stator magnetic pole has a concentrated winding 2. The rotor 4 is made of a plurality of magnetic steel sheets which are laminated. As has been well known, the switched reluctance motor has a solid structure and can operate at high rotation speed. Because the switched reluctance motor has no magnet, it can operate under high temperature conditions. Further, because the switched reluctance motor can be made of steel and copper, its manufacturing cost is low. Still further, because the winding can be wound on the stator 1 at the outside of the housing of the switched reluctance motor and the stator 1 with the winding is then assembled in the housing, easily-shapeable aluminum wires can be used. As a result, the switched reluctance motor made of steel and aluminum is easily recyclable.
For example, following four related art documents have disclosed such a type of the switched reluctance motor. Japanese patent laid open publications No. JP 2000-350390, JP 2001-186693, JP 2002-10593, and JP 2002-253896.
However, those conventional switched reluctance motors include the following drawbacks to be improved. In the following explanation, it is defined that a circumferential pitch of the projecting magnetic poles in a rotor is an electrical angle π.
(1) Because the motor has a plurality of discontinuous points at which a motor torque is generated, a torque ripple is often generated. This torque ripple would cause rotational vibration of the motor.
(2) Because each phase in the motor generates a torque power in order when a multi-phase motor drives, the generation of torque power in one of the stator magnetic poles and no generation of torque power in the other stator magnetic poles are simultaneously present on the circumference of the stator. This generates an unbalanced attraction force which is then applied to each magnetic pole. The stator is thereby deformed. The deformed stator further causes vibration and noise during its operation.
(3) In order to avoid the above drawbacks (1) and (2), it is possible to increase the number of the stator magnetic poles. Although this can decrease the magnitude of the attraction power applied to each magnetic pole, the increasing of the stator magnetic poles becomes difficult to keep the necessary space of the stator winding.
(4) Although extending each width of the adjacent stator magnetic poles in the circumferential direction can smoothly shift the motor torque between the stator magnetic poles, there is a possibility of generating a negative torque at the connection node between the adjacent stator magnetic poles if the stator magnetic pole width or a rotor magnetic pole width is not less than 90°. Thus, there is a limit to extend the stator magnetic pole width in order to decrease the fluctuation of the torque power. Considering from such a viewpoint, the conventional switched reluctance motor has the stator magnetic pole width and the rotor magnetic pole width of approximately 60°, as shown in FIG. 28.
There has been proposed another type of a brushless DC motor having a three-dimensional magnetic path structure. However, such a type of brushless motor generates a large amount of eddy current loss by a magnetic flux that flows in the axial direction when a laminated magnetic steel plate is used. In order to avoid this drawback, another conventional technique has proposed a soft magnetic powder material such as a powder magnetic core instead of using magnetic steel sheets that are laminated. However, because this structure of the conventional technique decreases a saturation magnetic flux, the motor characteristic of the brushless motor is thereby decreased.